PROJECT SUMMARY Human parvovirus B19 (B19V) is pathogenic to humans. It is highly infectious and causes a wide range of pathological conditions including fifth disease in children, persistent anemia in immunocompromised patients, transient aplastic crisis, and hydrops fetalis in pregnant women. Currently, neither a vaccine nor specific antiviral therapeutics is available to prevent or treat B19V-caused diseases. B19V exhibits a narrow tropism and its pathogenesis is restricted to human erythroid progenitor cells of the bone marrow and the fetal liver. P- antigen (globoside) is the primary receptor of B19V. Although ?5?1 integrin and Ku80 autoantigen have been proposed as co-receptors, their roles in the entry of B19V into human erythroid progenitor cells are uncertain. The B19V capsid consists of 60 structural subunits, of which ~95% are VP2 and ~5% are VP1. VP1 is identical to VP2 with the exception of an additional N-terminal region of 227 amino acid residues referred as the VP1 unique region (VP1u). VP1u is the immunodominant region of the capsid, containing major neutralization epitopes. Alone, VP1u binds to the cell surface of B19V-permissive cells and is internalized into the cells. The VP1u binding and internalization correlate with the extraordinarily narrow tissue tropism of B19V, more so even than the primary receptor P-antigen. Upon binding to P-antigen, the B19V virion changes its capsid structure by extruding VP1u outside the five-axis cylinder to interact with the cell surface, which is required for virion internalization. Importantly, VP1u does not interact with ?5?1 or Ku80 in vitro. Moreover, neither purified ?5?1 nor Ku80 blocked B19V infection of human erythroid progenitor cells. We hypothesize that a highly restricted, unknown co-receptor recognized by VP1u mediates B19V virion high affinity binding and internalization, which causes the narrow tropism of B19V. In this proposal, we aim to identify the VP1u receptor that binds and internalizes VP1u using biochemistry approaches, and to develop antiviral peptides derived from the VP1u domain responsible for VP1u receptor recognition. Identification of the relevant molecules and dynamic interactions that lead to B19V entry into human erythroid progenitor cells is essential to understand its extraordinarily restricted tropism, and the basis of viral pathogenesis. This understanding will form basis for developing efficient antiviral strategies that interfere with the early steps of B19V infection.